Terrain, add Texture

Hi, currently i use the Terrain Class from HelloTerrain as my map, customized ofc.
My simple Question for today is: How can i add more Textures? There are three (Tex1-Red, Tex2-Blue, Tex3-Green) in the engine already, but how can i make a new one, based on a color?

The Tutorial says “you see that you have several texture layers to paint on: Tex1, Tex2, Tex3, etc.” but if i look into the specific File, only the 3 textures re named,

Terrain lighting is the material you want to use: